1. Field of the Invention
The present invention relates to a lapped steel tube which is arranged generally as an oil or air feed passage or a Diesel engine fuel injection pipe in automobiles or various kinds of machines or apparatus and which has a diameter of about 20 mm or less and a pressure resistance and mechanical strength.
2. Prior Art
The lapped steel tube of this kind is generally demanded to have stabilities in qualities such as fatigue strength or sealing properties. Especially, the Diesel engine fuel injection pipe is demanded to have resistances to repeated high pressure fatigues, cavitation erosion and vibrations from the engine or car body. The lapped steel pipe satisfying such demands has been exemplified by a construction in which inner, outer and intermediate pipes are made of duplex-wound pipes and soldered to one another or a construction in which the inner pipe is made of a seamless pipe whereas the outer and intermediate pipes are made of duplex-wound pipes and soldered to one another.
However, the lapped steel tube of the prior art thus constructed has defects, as will be described in the following.
The lapped steel tube having the inner, outer and intermediate pipes made of the duplex-wound pipes can be manufactured at a low cost. However, the individual pipe members are press-fitted and drawn so that they may be lapped, and the copper plating film, as applied in advanced to the whole lapped faces, is heated and soldered. As a result, the parent material is deteriorated by the twice soldering and heating treatments at the time of making the duplex-wound pipes and at the time of shaping the lapped steel pipes. At the same time, the SPCC (low-carbon steel), as used in the duplex-wound pipe in view of its excellent compactibility, has a low tension resistance and a low vibration fatigue strength so that the outer surface, from which the vibration fatigue is started by the vibrations to be applied for use, is liable to break and to have an inferior vibration resistance. Moreover, a step is formed, although small, on the axial mating seam (or seam portion) to be formed when the duplex-wound pipes are made, and may be left on the sealing face of the joint head formed by a pressing treatment, to deteriorate the sealing properties. As a result, a problem arises when the duplex-wound pipe is used as the outer pipe. When the soldering of the whole abutment face is insufficiently executed when the lapped steel pipe is to be formed, the abutting circumferential faces, as press-fitted to each other, are loosened to leave a clearance. As a result, the inner pipe is repeatedly expanded and shrunk by the abrupt pressure fluctuation or temperature change of the fluid (e.g., a fuel) to flow in the inner pipe. This raises another problem that the a resistance to the fatigue due to the repeated internal pressures is liable to drop. When the inner pipe is made of a seamless pipe, on the other hand, the resistance to the pressure fatigue and the leakage prevention are high, but the aforementioned problem of the duplex-wound pipe itself cannot be solved. Another problem arises in the higher cost than that of the duplex-wound pipe or the electroseamed pipe.